1. Field of the Invention
The present invention relates to a process for the preparation of certain silicone elastomers upon addition of certain alkoxylated organosilicon cross-linking agents, together with certain metal catalysts, to certain storage-stable polysiloxane compositions.
More especially, this invention relates to the reinforcing of such polysiloxane compositions with silica fillers having high specific surface area, and which have been treated, in the presence of water, with certain Si-N bonded silylating agents, in such diorganopolysiloxane oils having viscosities in the range of from between about 400 to 3,000 cps at 25.degree. C., said reinforcing fillers and the various parameters for the treatment thereof being selected from within certain well-defined limits.
The organopolysiloxane compositions thus obtained are storage-stable and, in particular, their viscosities, which are within the range of from about 10,000 cps at 25.degree. C. to 150,000 cps at 25.degree. C., do not change with time; furthermore, even one hour after the addition of the aforementioned cross-linking agents and metal catalysts thereto, same can still readily be cast and are capable therefore of reproducing even the minutest details of the surfaces upon which they have been deposited or cast. The viscosities of the subject compositions, at the end of the noted post-addition period, attain a value at most one and one half times the corresponding value prior to addition of the curing agents.
2. Description of the Prior Art
Numerous processes are known to the state of the prior art which relate to the treatment of finely divided silicas with silazanes or silylamines with a view to improving the properties of the curable organopolysiloxane compositions [and/or elastomers] containing such siliceous fillers.
In general, the silicas are treated hot with a combination of additives which [in addition to the silazanes or the silylamines] comprise water, cyclopolysiloxanes and, optionally, organic amine compounds; this treatment can take place in one or plural stages [compare German published applications Nos. 2,358,784 and 2,359,619]. Given the small amounts of these various additives employed relative to the apparent volume of the silicas to be modified, it is essential, in order to suitably impregnate these pulverulent silicas, to employ bulky leakproof apparatus which is resistant to pressures greater than atmospheric pressure. Furthermore, once same have been impregnated, these silicas are necessarily removed from the aforedescribed apparatus and then transferred into other devices in order to mix them with the necessary organopolysiloxane polymers; while such an operation is technically feasible, it is time-consuming and, further, requires adherence to strict limits because it is essential to prevent the treated silicas from becoming dispersed in the air in the work area. The aforesaid disadvantages thus severely limit the use of processes of this type.
According to another technique [U.S. Pat. No. 3,642,685 and British Patent Specification No. 1,024,234], it is possible to treat siliceous or non-siliceous inorganic fillers directly with Si-N bonded organosilicon compounds in diorganopolysiloxane polymers having various viscosities. This process avoids the above-mentioned disadvantages but, nevertheless, it only permits organopolysiloxane compositions having viscosities which do not vary with time to be produced with great difficulty because of the proportions of the reactants required to be present or even because of the choice of the diorganopolysiloxane polymers themselves. Additionally, if compositions of this type are prepared they are only very rarely able to flow or otherwise be worked one hour after the addition of appropriate cross-linking agents and catalysts thereto.